Implantable defibrillator systems deliver a high voltage electrical countershock to the heart in an attempt to correct or convert a detected cardiac arrhythmia or fibrillation. Due to the limitations on size and power imposed by the fact that these systems must be self-contained implantable devices, all existing implantable defibrillator systems generate an electrical countershock by charging a capacitor system to a high voltage from a low voltage battery and oscillator circuit. The battery is then switched out of the circuit and the electrical charge stored in the capacitor system is delivered as a truncated capacitive discharge through two or more implanted electrodes.
To date, there have been two basic kinds of discharge waveforms which have been used with implantable defibrillator systems: monophasic waveforms and biphasic waveforms; both of which are delivered as a truncated capacitive discharge. Monophasic waveforms are comprised of a single monotonically decaying electrical pulse that is typically truncated before the capacitor system is completely discharged. Biphasic waveforms, on the other hand, are comprised of a decaying electrical pulse that has a pair of decaying electrical phases that are of opposite polarity. To generate a biphasic pulse an H-bridge switch circuit connected to the electrodes is used to switch the polarity of the two phases. In generating the biphasic pulse, a first phase is discharged from the capacitor system, much in the same manner as a monophasic pulse. At the point in time that the first pulse is truncated, the H-bridge switch circuit immediately reverses the discharge polarity of the capacitor system as seen by the electrodes to produce the second phase of the biphasic waveform that is of the opposite polarity. A typical example of the use of an H-bridge circuit to generate a biphasic waveform in an implantable defibrillator system is shown in U.S. Pat. No. 4,998,531.
Over the last twenty five years, it has been demonstrated that appropriately truncated biphasic waveforms can achieve defibrillation with significantly lower currents, voltages and energies than monophasic waveforms of similar durations. Kroll, M W et al., "Decline in Defibrillation Thresholds", PACE 1993; 16#1:213-217; Bardy, G H et al., "A Prospective Randomized Evaluation of Biphasic vs. Monophasic Waveform Pulses on Defibrillation Efficiency in Humans", J American College of Cardiology, 1989; 14:728-733; and Wyse, D G et al., "Comparison of Biphasic and Monophasic Shocks for Defibrillation using a Non-Thoracotomy System", American J Cardiology 1993; 71:197-202. These findings are of particular importance for implantable devices because of the direct relationship between the amount of energy required for defibrillation and the overall size of the implantable device, i.e., the lower the energy required for defibrillation, the smaller the device.
Numerous theories have been advanced to explain the improved efficiency of the biphasic waveform over the more conventional monophasic waveform. Although some of these theories may partly explain, or may act cooperatively to explain, the effect a biphasic waveform has on the heart, there is currently no single accepted theory which fully explains the advantages of the biphasic waveform over the monophasic waveform. As a result, there is little or no agreement on what factors might further improve the efficiency and operation of the biphasic waveform.
In terms of the circuitry used to generate biphasic waveforms, the conventional H-bridge switch circuit has proven to be the only circuit used to generate biphasic waveforms in existing manufactured implantable cardioverter defibrillator (ICD) systems. Unfortunately, the switching components required for the conventional H-bridge circuit have several drawbacks. First, there is some question as to the long-term reliability of the switching components when used for this application. Second, the switching components are relatively large and their use tends to decrease the space in the ICD system that would otherwise be available for batteries or capacitors.
In addition, the use of the conventional H-bridge circuit requires a capacitor system that will have sufficient residual charge after truncation of the first phase to adequately power the second phase of the biphasic pulse. With existing ICD systems, all of which have capacitor systems with effective capacitance values of 140 .mu.f or greater, this has not been a problem. The capacitor system is charged from a low voltage to high voltage energy converting system using a battery and flyback transformer combination oscillated at high frequency by a switching means and rectified to place a high voltage charge on the capacitor system. When sufficient energy has been stored, the energy converting system is turned off prior to delivery of the biphasic defibrillation pulse. As ICD systems are developed with smaller capacitance values, however, the amount of residual charge left in the capacitor system may not be sufficient to adequately power the second phase of the biphasic pulse.
The only other circuit design that has been proposed for generating biphasic waveforms arose out of the early theory that biphasic countershocks were more efficient because the net electrical charge transport was zero for a perfectly symmetrical biphasic waveform where the second phase was an exact mirror of the first phase. Schuder, J C et al. "Transthoracic Ventricular Defibrillation with Square-Wave Stimuli: One-Half Cycle, One Cycle and MultiCycle Waveforms", Circulation Research, 1964; 15:258-264. In recent experiments testing this theory, two identical capacitors were used to generate a "double capacitor" biphasic waveform in which a separate capacitor was used for each phase producing a symmetric biphasic waveform to ensure complete symmetry of the waveform. The results of these experiments established that mirror image dual capacitor systems were inferior to a single capacitor system in terms of producing lower &fibrillation threshold energies for symmetric biphasic waveforms. Kavanagh, K M et al., "Comparison of the Internal Defibrillation Thresholds for Monophasic and Double and Single Capacitor Biphasic Waveforms", J American College of Cardiology, 1989; 14:1343-1349; and Freeser, S A et al., "Strength-Duration and Probability of Success Curves for Defibrillation with Biphasic Waveforms", Circulation, 1990; 82:2128-2141.
While existing implantable defibrillator systems are capable of generating electrical countershocks that utilize the more efficient biphasic waveform, there presently is no single accepted theory for why the biphasic waveform is more efficient. This lack of an understanding of the nature and effect of the biphasic waveform has impeded further development and enhancement of the biphasic waveform. Accordingly, it would be desirable to provide a method and apparatus for generating biphasic waveforms for an implantable defibrillator system that overcomes the disadvantages of the existing H-bridge circuitry. It would also be advantageous to improve on the methods and apparatus for generating biphasic waveforms as a result of an improved understanding of the nature and effect of the biphasic waveform on the fibrillating myocardium.